Thermocouple apparatus and method

ABSTRACT

In a thermocouple, a pair of thermoelements extend within a protective sheath. The thermoelements are spaced from the sheath by an insulator. The sheath comprises an outer sheath formed from a metal alloy adapted to provide mechanical support and corrosion resistance during use of the thermocouple, typically at elevated temperature. The sheath further comprises an inner sheath positioned between the outer sheath and the thermoelements and formed from a nickel-based alloy containing less than 10 wt % Cr, to prevent diffusion of Cr and/or Mn from the outer sheath to the thermoelements.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national stage application of InternationalPatent Application No. PCT/GB2011/000506, filed Mar. 31, 2011, whichclaims priority to Great Britain Patent Application No. 1005509.3, filedMar. 31, 2010, the disclosures of each of which are incorporated hereinby reference in their entirety.

The invention relates to a thermocouple apparatus and method, and inparticular to a Mineral Insulated Metal Sheathed (MIMS) thermocouple.

MIMS thermocouples are widely used in industry and certain types havebecome popular as industry standards. For high-temperature applications,these include Type K and Type N thermocouples. However, at elevatedtemperatures above about 1000 C, conventional MIMS thermocouples sufferfrom two problems, namely oxidation (when used in oxidising atmospheres)and drift, i.e., a change of the measured voltage with time duringexposure of the thermocouple to high temperatures, which limits thereliability of temperature measurement. For conventional nickel-basedMIMS thermocouples, drift typically worsens above about 1000 C.Temperature measurement at temperatures above about 1000 C thendisadvantageously requires the use of much more expensive thermocouplesmade of noble metals such as platinum.

In a MIMS thermocouple, positive and negative thermoelements arecontained within a tubular metal sheath, and insulated from each otherand from the sheath by a compacted ceramic or mineral material (exceptat the thermocouple tip where the thermoelements are joined together andmay, in some thermocouple designs, be in electrical contact with thesheath). The sheath is designed to provide oxidation resistance, i.e.,to prevent oxidation of the thermoelements, but it can affect the driftbehaviour of the thermocouple. In particular, alloys which optimise theoxidation resistance of the sheath commonly contain manganese, and it isknown that migration of manganese from the sheath to the thermoelementsdue to diffusion at elevated temperature can change the Seebeckcoefficient of the thermoelements and so cause drift. This problem isdescribed, for example, in U.S. Pat. No. 5,043,023. This is a particularproblem with nickel-based thermocouples, and undesirably limits thehigh-temperature performance of these thermocouples.

BRIEF SUMMARY

In a first aspect, the invention may thus provide a thermocouple inwhich a thermoelement extends within a metal outer sheath, and anickel-based inner sheath containing less than 10 wt % of chromium ispositioned between the thermoelement and the metal outer sheath.

The term nickel-based means that the inner sheath comprises a nickelalloy, which contains nickel as the largest wt % component of the alloy,and is preferably more than 50 wt % Ni.

Advantageously, the metal outer sheath may serve to protect thethermoelement, and other components of the thermocouple within the outersheath, from the environment during operation of the thermocouple. Forexample, if the thermocouple is for use in an oxidising environment,then the outer sheath may advantageously protect the thermoelement fromoxidation. This is a common requirement, for example for thermocouplesoperating in air at elevated temperatures. For thermocouples operatingin other environments, the outer sheath may advantageously be designedto protect the thermoelement from those environments during operation.

It is known, as described above, that manganese contained in the outersheath of a conventional thermocouple may contaminate thermoelements andcause drift. In addition, the inventor has now appreciated that chromiumcontained in the outer sheath of a conventional thermocouple may alsocontaminate thermoelements and cause drift. Many conventional sheathsfor MIMS thermocouples are based on nickel-chromium alloys and thereforethe inventor has identified a significant problem. In practice,nickel-chromium alloys have properties which are highly desirable forthermocouple sheaths (for example, they show high-oxidation resistanceand are mechanically tough), but the inventor has appreciated that useof these materials may limit the high-temperature performance ofthermocouples by causing drift, even though they may providesatisfactory oxidation protection. In practice, it may not be possibleto exclude chromium from a thermocouple sheath alloy if adequateoxidation resistance is to be achieved.

In order to increase the maximum operating temperature of a conventionalMIMS thermocouple from, say, 1000 C to 1200 C or more, it is necessaryboth to ensure that the thermocouple is adequately protected from theenvironment (usually from oxidation) and that drift is prevented. A goodenvironmental (oxidation) resistance may be achieved by using aconventional oxidation-resistant alloy for the sheath, but in prior artthermocouples, contamination of the thermoelements by alloying elementsin such alloys has then limited the maximum operating temperature bycausing unacceptable drift. The diffusion barrier provided by the innersheath of the present invention may advantageously enable the maximumoperating temperature of a MIMS thermocouple to be increased bypreventing or reducing drift. Advantageously, embodiments of theinvention may allow operation of a MIMS thermocouple at 1100 C or more,or 1200 C or more. This may advantageously give operating temperatureranges from 1100 C or 1150 C or 1200 C up to 1250 C or 1300 C or 1350 Cfor example.

In various aspects of the invention, a number of materials are proposedfor the inner sheath. These include a pure nickel sheath, of greaterthan 99 wt % nickel or preferably greater than 99.5 wt % nickel. Anexample would be Nickel 270 (UNS N02270/W. Nr. 2.4050). Alternatively, anickel-based sheath may be used which does not contain chromium, orcontains chromium at less than 10 wt %, or less than 9 wt %, preferablyless than 5 wt %, particularly preferably less than 3 wt %, andparticularly preferably less than 1 wt %. In a specific embodiment, anickel-based sheath may be used that contains chromium at less than 0.1wt %.

A further preferred alternative is a nickel-based sheath which does notcontain manganese or has a manganese concentration lower than 0.2 wt %or 0.1 wt %, and also does not contain chromium or has a chromiumconcentration less than 10 wt %, less than 9 wt %, less than 5 wt % orless than 3 wt %.

A further aspect of the invention may advantageously provide athermocouple comprising a nickel-based inner sheath which does notcontain manganese or has a manganese concentration lower than 0.2 wt %,or preferably lower than 0.1 wt %, or particularly preferably lower than0.01 wt %. In this aspect of the invention the inner sheath may comprise10 wt % Chromium (Cr) or more. This inner sheath may be used wherecontamination of the thermoelement(s) with Magnesium (Mn) is to beavoided but where a higher concentration of Cr in the inner sheath, andtherefore the possibility of Cr contamination of the thermoelements, maybe acceptable.

Each of the inner sheaths described above is formed using a nickel-basedalloy comprising a limited concentration of Cr and/or Mn. As describedabove, the term nickel-based may mean that the alloy contains a highercontent (in wt %) of Nickel (Ni) than of any other element. In view ofthis, the skilled person would be able to fabricate inner sheathsembodying the invention, taking into account the thermal and mechanicalproperties required for the inner sheath of a thermocouple designed foroperation at 1100 C or above, or between 1100 C and 1200 C or even 1300C or 1350 C.

In a first preferred embodiment, nickel may form the whole of thebalance of the alloy composition except for the Cr and/or Mn contentdescribed above, or Ni may form at least 80%, 90%, 95% or 98% of thebalance of the alloy composition. Alternatively, the nickel content ofthe alloy may be greater than 50 wt %, 60 wt %, 70 wt %, 80 wt %, 85 wt% or 90 wt %.

Where Ni and Cr and/or Mn do not constitute the only components of thealloy, the balance may comprise one or more of the following:

-   -   Fe and/or Co, each up to 49 wt %, or preferably up to 40 wt %,        30 wt %, 20 wt % or 10 wt %;    -   Mo up to 25 wt %, or preferably up to 15 wt %, 10 wt % or 5 wt        %;    -   W up to 15 wt %, or preferably up to 10 wt % or 5 wt %;    -   Nb, Ta, V, Ti, Al, Si, Mg, Cu and/or Hf each up to 5 wt %, or        preferably up to 3 wt %, 2 wt % or 1 wt, up to a cumulative        maximum for these elements of 15 wt %, or preferably up to 10 wt        % or 5 wt %.

In all cases, as the skilled person would appreciate, the alloy maycomprise other trace elements or contaminants at acceptably lowconcentrations in known manner without affecting the performance of thealloy.

In an embodiment of the invention, a thermocouple may comprise a pair ofthermoelements extending within an inner sheath, with the outer sheathencircling the inner sheath. The inner sheath and the outer sheath maybe in the form of a pair of concentric or coaxial tubes, which may be incontact with each other. In a preferred embodiment, the inner sheath andthe outer sheath may take the form of a double-walled sheath structure,the inner and outer sheaths optionally being formed simultaneously bycoextrusion or swaging.

In alternative embodiments, the inner sheath and the outer sheath may bespaced from one another, and optionally insulated from one another.

A thermocouple comprises two or more thermoelements and in aspects ofthe invention one, two or more thermoelements may extend within the sameinner sheath.

In another embodiment, two thermoelements may each extend within one oftwo separate inner sheaths. In this structure, each inner sheath mayadvantageously provide a diffusion barrier for its respectivethermoelement, not only preventing diffusion of elements from the outersheath, but also preventing contamination of one thermoelement bydiffusion of elements from the other thermoelement.

In embodiments of the invention the thermocouple is preferably a nickelbased thermocouple. In such a thermocouple the thermoelements arenickel-based thermoelements, or are formed of Ni alloys, that is alloyswhich contain Ni as the largest wt % component. Often such alloys maycomprise more than 50 wt % Ni.

In each case, the provision of the inner sheath as a diffusion barrierbetween the outer sheath and one or more thermoelements mayadvantageously enable the design of the outer sheath to be optimised forenvironmental protection of the thermocouple, without the constraint ofavoiding the use of manganese and/or chromium in the outer sheath, whichcause thermoelement contamination and drift at elevated temperatures dueto the effect of these contaminants in changing the Seebeck coefficientof the thermoelements.

A conventional MIMS thermocouple such as a Type K or Type N thermocouplemay be usable to measure temperatures up to about 1000 C, but ifconventional thermocouple sheaths are used, these thermocouplestypically suffer from increasing amounts of drift at highertemperatures, such as 1100 C, 1200 C, or 1300 C. It may be possible tofabricate sheaths for MIMS thermocouples using conventional alloys (suchas Inconel 600) to provide oxidation resistance at these highertemperatures but these alloys contain elements which then lead tocontamination of the thermoelements at temperatures above about 1000 C,causing drift. Inconel 600 is a commonly-used material for conventionalthermocouple sheaths, which contains up to 1 wt % Mn and usuallycontains between 0.3 wt % and 0.5 wt % Mn. Provision of a diffusionbarrier in the form of an inner sheath, according to the aspects of theinvention described, may advantageously overcome the problem ofthermocouple drift when high-temperature-resistant alloys (such asInconel 600) are used for an outer sheath and enable operation ofthermocouples such as Type K or Type N thermocouples, or otherthermocouples, including in particular nickel-based thermocouples, at orabove higher temperatures, such as 1100 C, 1200 C or 1300 C.

The thickness of the inner sheath needs to be sufficient for the sheathto function as a diffusion barrier or contamination barrier. Forexample, the sheath thickness may be about 300 micrometers.

The or each thermoelement in each embodiment of the invention iselectrically insulated from the or each inner sheath and each otherthermoelement, at least along the length of the thermocouple, asrequired for functionality of the thermocouple. For example, aninsulating material may space and insulate the or each thermoelementfrom the or each inner sheath. For the avoidance of doubt, thisreference and other references in this document to the thermoelement(s)being spaced and/or insulated from the inner sheath refer to the lengthof the thermocouple, or thermocouple wire, excluding the thermocoupletip. At the thermocouple tip, thermoelements of the thermocouple arejoined together, in electrical contact, and in some thermocouple designsthe thermoelements may be electrically connected to the outer and/or theinner sheath(s) at the thermocouple tip. For example, in a thermocouplehaving an ungrounded configuration, the thermoelements may be connectedto each other but not to the inner or outer sheath at the thermocoupletip, while in a grounded configuration the thermoelements areelectrically connected to the outer and/or the inner sheaths at thethermocouple tip, and the respective inner and/or outer sheaths may beelectrically connected to ground or earth.

In implementations of the invention, the outer sheath may be fabricatedfrom any suitable material, without needing to avoid elements which maycontaminate the thermoelements. The outer sheath may be nickel-based, ormay be based on some other material. In conventional MIMS thermocouples,the sheaths are fabricated by conventional methods for tube fabrication,such as extrusion and/or drawing. In embodiments of the presentinvention, the inner and outer sheaths may similarly be fabricated byconventional methods for tube fabrication. In a similar way to aconventional MIMS thermocouple, an assembly of the inner and outersheaths, the thermoelements and the insulating material (such as aceramic or mineral material) can then be extruded or swaged down to arequired size, or diameter, to form a MIMS cable. During this process,the thermoelements and insulation may be fabricated in the same way asfor a conventional thermocouple, as the skilled person would appreciate.

To form a thermocouple, currently the end of a length of MIMS cable isusually crimped and welded (TIG welded usually) so that a sealed end isproduced. This straightforward method applied to a double sheaththermocouple embodying the invention may result locally at the tip in analloy whose concentration is lower in chromium (and for this reason lessoxidation resistant) because of mixing between the low Cr content innersheath and higher Cr content outer sheath. Three possible sealingstrategies to solve this problem are as follows:

-   -   Removal of the inner sheath at the end to be sealed by drilling        and sealing of the outer sheath with standard prior art        technology. Once the MIMS cable has been cut, at its open end a        drill or cutting tool of diameter equal to the external diameter        of the inner sheath can be used to remove a predetermined length        of the inner sheath, leaving only the outer sheath at the end of        the cable. An annular drill or cutting tool may be used to        prevent damage to the ends of the thermoelements within the        sheath. The end portion of the outer sheath can then be, for        example, crimped and welded without affecting its alloy        composition.    -   Using a filler metal with higher Cr content than the outer        sheath: a small volume of the filler metal is located, or        inserted, in the gap at the open end of the MIMS cable and then        the end of the MIMS cable is welded. The composition and mass of        the filler metal are chosen to compensate for the mixing between        the lower Cr content inner sheath and the higher Cr content        outer sheath.    -   Crimping of the inner sheath and welding of the inner sheath        followed by sealing of the outer sheath. This may involve        crimping of the outer sheath and welding with lower power to        achieve welding of the outer sheath but not remelting of the        inner sheath. Alternatively, a filler metal may be provided        before welding of the outer sheath with lower power to achieve        welding of the outer sheath and filler metal but not remelting        of the inner sheath.

In the embodiments of the invention described above, in which anassembly of inner and outer sheaths is formed into a MIMS cable byextrusion and/or drawing and/or swaging, the inner and outer sheaths maytypically extend along the entire length of a thermocouple. In manycases this is satisfactory and provides an effective way to fabricatethermocouples, but to achieve the benefit of the inner sheath inpreventing contamination of the thermoelements by diffusion of materialfrom the outer sheath, in an alternative embodiment it may only benecessary for the inner sheath to extend along the portion of thethermocouple which is to be exposed to high temperatures during use.

It may be noted that in some of the techniques described above forsealing an end of a thermocouple cable, a small portion of thethermoelements may not be shielded by the inner sheath from the outersheath at the end of the thermocouple. For example if an end portion ofthe inner sheath is removed and the end of the outer sheath is crimpedand welded, or if a filler metal (such as a high Cr-content fillermetal) is provided and the inner and outer sheaths are crimped andwelded at the same time, then the inner sheath may not be continuous ormay have an open end. In that case the portions of the thermoelements ator near to the end of the thermocouple may be disadvantageously notentirely shielded by the inner sheath. In practice, this is unlikely tohave a significant effect on the efficacy of the inner sheath as all ofthe length of the thermoelements extending within the portion of thethermocouple exposed to high temperature during use is shielded by theinner sheath except for a small portion at the thermocouple tip.

As the operating temperature of a MIMS thermocouple is increased aboveabout 1000 C, the oxidation resistance provided by the sheath may nolonger be adequate. A further aspect of the invention therefore providesthat at least a portion of the outer surface of the sheath may be coatedwith an intermetallic or coated using chromizing. Advantageously, theintermetallic may be a nickel-aluminide or platinum-dopednickel-aluminide. The coating may preferably only be applied to theportion of the thermocouple sheath which will be exposed to hightemperatures.

For example, in a preferred embodiment of the invention a thermocouplecomprising an outer sheath and an inner sheath (functioning as acontamination barrier as described above) may be allowed to operate at astill higher temperature if at least a portion of the outer surface ofthe outer sheath is coated with an intermetallic or coated usingchromizing as described above.

Although applying a temperature-resistant coating to the outer sheath ofa thermocouple comprising an outer sheath and an inner sheath may thusbe particularly advantageous, this aspect of the invention may also beused to enhance the performance of a sheath of a thermocouple which doesnot comprise an inner sheath, but which only comprises a single sheath.For example, if an intermetallic coating or a coating obtained bychromizing is to be used, then an alloy for a sheath may be selectedwhich does not contain elements that contaminate the thermoelements at aproposed operating temperature, but which does not provide adequateenvironmental protection at that proposed operating temperature. Forexample, any of the alloys described above for fabricating the innersheath of the thermocouple according to the previous aspects of theinvention could be used. The environmental protection (e.g., oxidationresistance) provided by the sheath may then be enhanced by coating atleast a portion of the outer surface of the sheath (preferably theportion of the outer surface which will be exposed to high temperaturesduring use of the thermocouple) with an intermetallic or usingchromizing, such that the coated sheath does provide adequateenvironmental protection at the proposed operating temperature. In thisway, a thermocouple capable of operating at elevated temperatures may beprovided, such as a temperature at or above 1100 C, 1200 C, 1300 C oreven 1350 C.

Embodiments of the invention may be used in any industrial temperaturemeasurement application suitable for thermocouples, such as gas turbine,high-temperature process control, furnace temperature control and so on.Advantageously, however, aspects of the invention may enableconventional types of thermocouple, such as Type K and Type Nthermocouples, to operate at higher temperatures than previously.

A specific embodiment is directed to a thermocouple, in which athermoelement extends within an outer sheath, comprising an inner sheathpositioned between the thermoelement and the outer sheath, in which theinner sheath is adapted to prevent diffusion of contaminants which ifabsorbed by the thermoelement would change its Seebeck coefficient, fromthe outer sheath to the thermoelement during exposure of thethermocouple to elevated temperatures. In a further specific embodiment,the inner sheath of the thermocouple is a nickel-based inner sheathcontaining less than 10 wt % of chromium.

BRIEF DESCRIPTION OF THE DRAWING

Specific embodiments of the invention will now be described by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1A is a schematic transverse section of a thermocouple according toa first embodiment of the invention;

FIG. 1B is a schematic longitudinal section of the thermocoupleaccording to the first embodiment of the invention, schematicallyshowing the tip portion of the thermocouple;

FIG. 2A is a schematic transverse section of a thermocouple according toa second embodiment of the invention;

FIG. 2B is a schematic longitudinal section of the thermocoupleaccording to the second embodiment of the invention, schematicallyshowing the tip portion of the thermocouple;

FIG. 3A is a schematic transverse section of a thermocouple according toa third embodiment of the invention; and

FIG. 3B is a schematic longitudinal section of the thermocoupleaccording to the third embodiment of the invention, schematicallyshowing the tip portion of the thermocouple.

SPECIFIC EMBODIMENTS AND BEST MODE OF THE INVENTION

FIG. 1 is a transverse section of a thermocouple comprising twothermoelements 2, 4 extending within a composite sheath 6. Thethermoelements are as in a conventional thermocouple, such as a Type Kor Type N thermocouple. Along the length of the thermocouple, thethermoelements are insulated from each other and from the inner surfaceof the sheath by an insulating ceramic material 8, in the same way as ina conventional thermocouple.

The sheath is tubular and comprises an inner sheath 10 and an outersheath 12. The outer sheath is of a conventional oxidation-resistantalloy such as Inconel 600, but may be of any conventionalenvironment-resisting alloy. The inner sheath is of nickel-based alloy270 (Nickel 270), but may be of any of the nickel-based compositionsdescribed above. The thermocouple is intended for operation at elevatedtemperature, such as above 1000 C. In this temperature range, in aconventional thermocouple the manganese and chromium content in theInconel 600 would cause contamination of the thermoelements andconsequently cause drift. In the embodiment, the inner sheath provides adiffusion barrier and prevents contamination of the thermoelements byeither the manganese or chromium in the outer sheath.

The outer sheath is of similar dimensions to a conventional thermocouplesheath, both in terms of diameter and thickness. The inner sheath isbetween 300 and 500 micrometers thick.

FIG. 2 is a transverse section of a thermocouple comprising twothermoelements 52, 54 extending within an outer sheath 56. Eachthermoelement is contained within a respective inner sheath 58, 60.Along the length of the thermocouple, each thermoelement is insulatedfrom its respective inner sheath, and each inner sheath is insulatedfrom the other inner sheath and from the outer sheath, by an insulatingceramic material 62. The materials of the thermoelements, the inner andouter sheaths and the insulating material may be the same as in thefirst embodiment.

As in the first embodiment, the inner sheaths provide a contaminationbarrier to prevent contamination of the thermoelements by elements suchas manganese and chromium in the outer sheath. In addition, eachthermoelement is separated from the other thermoelement by the innersheaths, and so any contamination of one thermoelement by diffusion ofelements from the other is prevented.

FIG. 3 is a transverse section of a thermocouple having a similarstructure to the thermocouple of FIG. 1. Reference numeralscorresponding to those in FIG. 1 will therefore be used. Thethermocouple of FIG. 3 comprises two thermoelements 2, 4 extendingwithin a composite sheath 6 comprising an inner sheath 10 and an outersheath 12. Along the length of the thermocouple, the thermoelements areinsulated from the inner sheath and from each other by an insulatingceramic material 8. The outer surface of the outer sheath is coated witha layer of a nickel-aluminide or a platinum-doped nickel-aluminide 14 ofthickness approximately 100 to 200 micrometers. Alternatively, the outersurface of the outer sheath can be coated by chromizing. The coatingenhances the oxidation resistance of the outer sheath.

The nickel-aluminide or platinum-doped nickel-aluminide coating, or thecoating produced by chromizing, is particularly effective in combinationwith the outer sheath, in providing an oxidation-resistant andprotective sheath for the thermocouple. The outer sheath is typicallymade of a nickel-chromium alloy or nickel-chromium-aluminium alloy andthe coating bonds well to these alloys. The coating is typically moreexpensive than a nickel-chromium alloy sheath material and therefore isadvantageously selectively used to coat only the portion of thethermocouple which will be exposed to elevated temperatures during use,i.e. the portion of the thermocouple sheath close to the junctionbetween the thermoelements.

The invention claimed is:
 1. A thermocouple, comprising: a cable portionof the thermocouple; and a tip portion of the thermocouple, wherein thecable portion of the thermocouple comprises: a metal alloy outer sheath;a first thermoelement, wherein the first thermoelement extends withinthe metal alloy outer sheath; a second thermoelement, wherein the secondthermoelement extends within the metal alloy outer sheath; and anickel-based inner sheath containing less than 10 wt % chromium, whereinthe nickel-based inner sheath contains greater than 90 wt % nickel, andwherein the nickel-based inner sheath is positioned between the firstthermoelement and the metal alloy outer sheath, and wherein the firstthermoelement and the second thermoelement are joined together inelectrical contact at the tip portion of the thermocouple.
 2. Thethermocouple according to claim 1, wherein the nickel-based inner sheathcontains greater than 99 wt % nickel.
 3. The thermocouple according toclaim 1, wherein the nickel-based inner sheath is formed from Nickel270.
 4. The thermocouple according to claim 1, wherein the nickel-basedinner sheath contains less than 5 wt % chromium.
 5. The thermocoupleaccording to claim 1, wherein the nickel-based inner sheath containsless than 0.1 wt % chromium.
 6. The thermocouple according to claim 1,wherein the nickel-based inner sheath contains less than 0.2 wt %manganese.
 7. The thermocouple according to claim 1, wherein thenickel-based inner sheath contains less than 0.01 wt % manganese.
 8. Thethermocouple according to claim 1, wherein the metal alloy outer sheathand the nickel-based inner sheath are in the form of coaxial tubes. 9.The thermocouple according to claim 1, wherein the metal alloy outersheath is in contact with the nickel-based inner sheath.
 10. Thethermocouple according to claim 1, wherein the cable portion of thethermocouple further comprises: a second nickel-based inner sheathcontaining less than 10 wt % chromium, wherein the second nickel-basedinner sheath is positioned between the second thermoelement and themetal alloy outer sheath.
 11. The thermocouple according to claim 1,wherein the first thermoelement is formed from Ni alloys, and wherein Niis the largest component by weight of the Ni alloys.
 12. Thethermocouple according to claim 1, wherein at least a portion of anouter surface of the metal alloy outer sheath is coated with anintermetallic or is coated using chromizing.
 13. The thermocoupleaccording to claim 12, wherein the at least a portion of the outersurface of the metal alloy outer sheath is coated with theintermetallic, and wherein the intermetallic is a nickel-aluminide orplatinum-doped nickel-aluminide.
 14. The thermocouple according to claim1, wherein the nickel-based inner sheath is adapted to prevent diffusionof contaminants, which if absorbed by the first thermoelement wouldchange a Seebeck coefficient of the first thermoelement, from the metalalloy outer sheath to the first thermoelement, along the cable portionof the thermocouple, during exposure of the thermocouple to temperaturesabove 1000° C.
 15. The thermocouple according to claim 8, wherein thecable portion of the thermocouple further comprises: an insulatingmaterial positioned: (i) between the first thermoelement and the secondthermoelement; (ii) between the first thermoelement and an inner surfaceof the nickel-based inner sheath; and (iii) between the secondthermoelement and the inner surface of the nickel-based inner sheath,such that: (a) the first thermoelement and the second thermoelement areinsulated from each other; (b) the first thermoelement is insulated fromthe inner surface of the nickel-based inner sheath; and (c) the secondthermoelement is insulated from the inner surface of the nickel-basedinner sheath.
 16. The thermocouple according to claim 1, wherein thethermocouple is configured to prevent oxidation of the firstthermoelement and the second thermoelement when the thermocouple isoperated in an oxidizing environment above 1000° C.
 17. The thermocoupleaccording to claim 14, wherein the metal alloy outer sheath contains:(i) manganese; (ii) chromium; or (iii) manganese and chromium, andwherein the nickel-based inner sheath is adapted to: (a) preventdiffusion of manganese: (b) prevent diffusion of chromium; or (c)prevent diffusion of manganese and chromium, respectively, from themetal alloy outer sheath to the first thermoelement, along the cableportion of the thermocouple, during exposure of the thermocouple totemperatures above 1000° C.
 18. The thermocouple according to claim 1,wherein the metal alloy outer sheath is formed of an oxidation-resistantmetal alloy.
 19. The thermocouple according to claim 14, wherein thenickel-based inner sheath has a thickness between 300 and 500micrometers.
 20. The thermocouple according to claim 10, wherein thecable portion of the thermocouple further comprises: an insulatingmaterial positioned: (i) between the first thermoelement and an innersurface of the nickel-based inner sheath; (ii) between the secondthermoelement and an inner surface of the second nickel-based innersheath; (iii) between an outer surface of the nickel-based inner sheathand an outer surface of the second nickel-based inner sheath; (iv)between the outer surface of the nickel-based inner sheath and an innersurface of the metal alloy outer sheath; and (v) between the outersurface of the second nickel-based inner sheath and the inner surface ofthe metal alloy outer sheath, such that: (a) the first thermoelement isinsulated from the inner surface of the nickel-based inner sheath; (b)the second thermoelement is insulated from the inner surface of thesecond nickel-based inner sheath; (c) the outer surface of thenickel-based inner sheath is insulated from the outer surface of thesecond nickel-based inner sheath; (d) the outer surface of thenickel-based inner sheath is insulated from the inner surface of themetal alloy outer sheath; and (e) the outer surface of the secondnickel-based inner sheath is insulated from the inner surface of themetal alloy outer sheath.
 21. A method of measuring a temperature,comprising: providing a thermocouple, wherein the thermocouplecomprises: a cable portion of the thermocouple; and a tip portion of thethermocouple, wherein the cable portion of the thermocouple comprises: ametal alloy outer sheath; a first thermoelement, wherein the firstthermoelement extends within the metal alloy outer sheath; a secondthermoelement, wherein the second thermoelement extends within the metalalloy outer sheath; and a nickel-based inner sheath containing less than10 wt % chromium, wherein the nickel-based inner sheath contains greaterthan 90 wt % nickel, and wherein the nickel-based inner sheath ispositioned between the first thermoelement and the metal or metal alloyouter sheath, and wherein the first thermoelement and the secondthermoelement are joined together in electrical contact at the tipportion of the thermocouple; interconnecting the thermocouple to anobject; exposing the thermocouple to an oxidizing environment at atemperature above 1000° C.; receiving a voltage from the thermocouple;and determining a measured temperature of the object via the voltage.22. A method for fabricating a thermocouple, wherein the thermocouplecomprises: a cable portion of the thermocouple; and a tip portion of thethermocouple, wherein the cable portion of the thermocouple comprises: ametal alloy outer sheath; a first thermoelement, wherein the firstthermoelement extends within the metal alloy outer sheath; a secondthermoelement, wherein the second thermoelement extends within the metalalloy outer sheath; and a nickel-based inner sheath containing less than10 wt % chromium, wherein the nickel-based inner sheath contains greaterthan 90 wt % nickel, and wherein the nickel-based inner sheath ispositioned between the first thermoelement and the metal alloy outersheath, and wherein the first thermoelement and the second thermoelementare joined together in electrical contact at the tip portion of thethermocouple, and wherein the method comprises: forming the metal alloyouter sheath and the nickel-based inner sheath simultaneously bycoextrusion or swaging; and joining the first thermoelement and thesecond thermoelement together in electrical contact at the tip portionof the thermocouple.